1. Field of the Invention
The present invention generally relates to methods and systems used to measure mechanical properties of materials. More particularly, the present invention relates to such methods and systems in which a penetration depth of an indenter is registered by a non-contact sensor configuration.
2. Description of the Related Art
It is well known that compliance affects the accuracy of depth measurements during hardness, scratch and/or wear testing. Compliance is generally used to describe displacement caused by deformation of parts of the instrument under load, and includes movement of the sample holder caused by loading during testing.
For instrumented indentation hardness testing of a sample, some have used a mechanical arm to touch a sample surface in order to generate a reference location for distance measurement. As the mechanical arm presses against a sample, the reference feeds back the height of the surface.
Because the reference is in mechanical contact with the surface, the accuracy of the data suffers. Also, the geometry of the contact point and the surface roughness of the sample can affect the accuracy of the test results. At the nanometric level, a surface roughness in the micron range will affect the accuracy of the final depth measured even if the claimed resolution of the sensor used (often a capacitor or linear variable differential transformer, or LVDT) is excellent.
Moreover, due to the mechanical contact between the mechanical arm and the sample surface, softer materials, such as plastics or polymers, for instance, cannot be accurately measured because the contact point sinks into the material or because material creep translates into movement of the reference as it deforms the sample being tested.
For scratch and wear testing, it is simply not possible to touch the surface with the same reference while the sample is moving. Therefore, the total plastic and elastic deformation depth during the scratch and wear testing is masked by the effects of compliance.
Because physical contact is limiting and cumbersome, testing samples of varying geometry can be difficult. The mechanical contact also takes more time because it needs to stabilize before testing can start.